It is characteristic of fixed capacity refrigeration systems that they include compressors driven by relatively inexpensive constant speed motors. Small air conditioning systems, typically of less than 10 ton capacity, are often of a single stage design having one compressor operating at constant capacity. Larger staged systems, up to about 100 ton capacity, may have a plurality of compressors each operating at its own fixed capacity so that the total output of the system can be varied in stages. The expression "fixed capacity" as used herein for refrigeration systems means both single and multiple stage systems. Also, the term "refrigeration systems" as used herein is meant to include air conditioning as well as other cooling systems.
The compressors of fixed capacity refrigeration systems are either operating at constant output or they are turned off. It is considered undesirable to cause a fixed capacity system to be shut down every time the load requirement drops below that constant cooling capacity of the system because once turned off the load requirement usually returns promptly to its higher level and the system must be restarted. Frequent on-off cycling of that sort results in a non-uniform cooling effect. In a building air conditioning system, for example, where the input air parameters change over time such cycling results in markedly variable temperature and humidity in the building.
Hence fixed capacity refrigeration systems are often allowed to continue in operation even when their capacity exceeds the load requirements. Part of the system capacity is simply wasted. One way to do this is by the use of a hot gas by-pass circuit in which heat normally discharged from the system is added back to the evaporator. This false load, then added to the reduced process fluid load, allows the system to operate continuously without overcooling the process fluid. These obviously undesirable techniques of wasting cooling capacity in a constantly running system can in many cases consume more than half the total energy input.
In my U.S. Pat. No. 5,372,011 there is described a fixed capacity cooling system where a phase change material (hereinafter referred to as a PCM) is frozen when the cooling load requirements are reduced. When the system is shut down and the reduced cooling load requirements continue they are suitably met by circulating a cooling fluid from heat exchange contact with the frozen PCM to heat exchange contact with the process fluid or load. This melts the PCM and when melting is complete the system is restarted. The compressor in such systems may be cycled on and off while the process fluid is continuously cooled at a reduced level without wasting energy.
The present invention is an improvement on the thermal storage system of my prior patent. It is intended especially for smaller refrigeration loads such as less than 10 tons capacity for single stage and less than 100 tons capacity for multiple stage compressor operation. In contrast with my earlier patented system, the present system is much simpler and does not involve circulation of a cooling fluid between the PCM thermal storage medium and the load. The PCM thermal storage medium in the present system is in direct heat exchange contact with the process fluid or load, for example in the input air duct of an air conditioning system for a building.
Placement of a thermal storage medium in relation to a process fluid flow so that an intermediary coolness fluid is unnecessary is not in itself novel. U.S. Pat. No. 5,054,540 describes one such arrangement but with a decomposable gas hydrate rather than a PCM, and not with a fixed capacity compressor or with refrigerant conduits passing through thermal storage containers as in the present system. U.S. Pat. Nos. 2,677,243 and 4,403,731 describe locating a PCM in a process fluid flow but for heat storage alone and with components unlike those of the present invention. U.S. Pat. No. 2,102,725 also teaches the use of a thermal storage material in contact with process air in a railway refrigerator car, but the material is brine and not a PCM and the compressor is not of fixed capacity. U.S. Pat. No. 2,884,768 also involves thermal storage in an air conditioning system but not with a fixed capacity compressor.